Final answer:
The tectonic plate west and adjacent to Montana is the Juan de Fuca Plate, which will eventually be subducted under the North American Plate, influencing the geology of the region including the Cascade Range.
Step-by-step explanation:
The name of the tectonic plate directly to the west and directly adjacent to the plate on which Montana is located is the Juan de Fuca Plate. This plate is located off the coast of Washington and Oregon in the northwestern United States. The Juan de Fuca Plate is bounded by several plate boundaries, including the Cascadia subduction zone to the east where it is being subducted under the North American Plate. This subduction is responsible for the tectonic processes that formed the Cascade Range, which includes volcanoes such as Mt. St. Helens. Given the plate boundary types and their movements, the ultimate fate of the Juan de Fuca Plate is to be completely subducted beneath the North American Plate over millions of years.